In the prior annual report we described our findings of non-replication of the reported genetic linkage of X-chromosome markers (Xg blood group antigen and protan/deutan colorblindness) to Bipolar (manic depressive) illness in informative pedigrees. We have this year combined our data on color blindness linkage with that of three other collaborating centers in the Biological Psychiatry Collaborative Program of the World Health Organization (WHO). One pedigree suggested linkage, 7 pedigrees suggested non-linkage, and 8 were indeterminate. We conclude that ascertainment or diagnostic differences among centers may be causing the data to be heterogeneous, although it is possible that a relatively small proportion of Bipolar illness may be in fact linked to color blindness.